Abstract
<title>Abstract</title> <p> <bold>Background</bold> Carex subg. Siderosticta is one of the six subgenera recognized in the recently revised infrageneric classification of Carex based on phylogenetic studies. It represents the earliest-diverging lineage within Carex and is sister to all remaining species. This morphologically diverse subgenus is distributed across East and Southeast Asia. Historically, the approximately 30 species of this subgenus were assigned to four morphology-based sections, namely Siderostictae, Hemiscaposae, Surculosae, and Hypolytroides. <bold>Results</bold> Phylogenetic analyses based on three plastid (matK, ndhF, and rps16) and two nuclear (ETS and ITS) DNA markers recovered the four individuals of Carex ensiformis as a strongly supported monophyletic lineage. Micromorphological observations revealed that C. ensiformis differs from the morphologically similar species C. damiaoshanensis in having slightly undulate anticlinal cell walls in the nutlet epidermal and a markedly shorter beak at the nutlet apex. Morphologically, the new species can be distinguished from C. damiaoshanensis by its more numerous inflorescence branches and spikes, as well as smaller utricles, nutlets, and shorter glumes. <bold>Conclusions</bold> Both morphological and molecular evidence reveal that Carex ensiformis is a new species belonging to subg. Siderosticta from southern China. The discovery of this species enriches our understanding of species diversity and taxonomic complexity within the subgenus, and emphasizes the importance of continued taxonomic studies in southern China. </p>